Industrial waste stream effluents present problems of purification which are quite different from the problems of municipal sewage treatment. The waste stream from each industrial process type is unique to that process and requires specialized rectification. pH control of acidic or alkaline waste materials, while of ecological importance, may be readily achieved in industrial waste streams by direct chemical treatment. The removal, solubilization or passivation of specific contaminants however, presents a problem more difficult of solution, which ofttimes require a combination of physical, biochemical and chemical treatments. Especially trying are the problems of preventing color, Chemical Oxidation Demand (COD), Biological Oxidation Demand (BOD) and chloride contamination of the surface water into which pulp mill waste streams are discharged. Color contamination of surface water presents an especially visible aesthetic problem as well as an adverse effect upon aquatic vegetable and animal life caused by altered light penetration of the surface water.
The purification of pulp mill effluents pose a unique problem in waste water treatment because the effluents are less amenable to the conventional water treatment procedures known and applied today in municipal and industrial waste water purification. The pulp production process creates multiple effluents having intense coloration which is largely the result of extracted tannins, lignins and their derivatives being removed from the cellulosic pulp, by selected chemical treatment stages. Any change in the actual processing of pulp which provides an effluent containing fewer contaminants, though carrying with it a reduced requirement for effluent treatment before discharge of the waste to surface water, usually adversely affects the quality of the pulp product. This first line of attack, the actual chemical bleaching technique applied, is then, restrictive of change as it poses the combined problems of economics and effectiveness of the chemical treatment, as well as the achievement of the desired bleached product characteristics demanded by the pulp and paper industry and is critical in any active program of water pollution abatement.
In conventional multi-stage bleaching sequences wherein chlorination (C), sequential chlorination (D.sub.c) or mixtures of chlorine and chlorine dioxide (D/C) are employed, the bulk of the contaminants include chlorides, lignins, tannins and their chlorinated and oxidized derivatives. By sequential chlorination (D.sub.c) is meant treatment with chlorine dioxide followed directly by treatment with chlorine; by chlorination (C) is meant treatment with chlorine without chlorine dioxide, and by mixtures of chlorine and chlorine dioxide (D/O is meant that treatment with chlorine and chlorine dioxide is simultaneous. Each of these contaminants is diverse in its reaction to specific pollution control mechanisms and each may be detrimental to the operation or economics of treatment stages tailored to effect one or more of the others. For example, chlorides rapidly corrode and foul equipment so that the transporting of such chlorides, with the organic contaminant stream, decreases the life and increases the cost of each treatment to which the stream is subjected. Accordingly, although chemical and physical treatment of the color bodies in pulp mill effluents has been an effective technique for reducing the amount thereof, the economics of such systems have been heretofore exorbitant and impractical in modern-day technology so that such techniques have not generally been applied to pulp mill effluents in practice with few exceptions. The use of activated sludge facilities, aerated lagoons and trickling filters have been successfully employed to decrease the BOD organics to more than 80% effectiveness but only remove from between about 10 to 15% of the color in the waste water effluent from a pulp bleaching plant. Accordingly, between 85 and 90% of the colored material from bleaching plant will then pass through the biochemical degradation step into the surface water disposal.
Conventional bleach sequences employed as multi-stage bleaching processes for wood pulp typically involve the treatment of the cellulosic pulp with chlorine (C), sequential chlorination (D.sub.c) or mixtures (D/C), followed by a caustic extraction (E) or oxygen (O) treatment stage. In practice, further chlorine dioxide, caustic extraction, oxygen or peroxide stages may be included within the total process. Two particularly effective multi-stage bleach processes are conventionally designated CEDED and D.sub.c EDED the letters, respectively, representing (C) chlorine, (D.sub.c) sequential chlorine dioxide/chlorine, (E) extraction and (D) chlorine dioxide treatment stages. Other effective multi-stage bleaching processes include D/CEDED, D.sub.c ODED, D/CODED, D.sub.c ED, D/CED, D.sub.c OD, D/COD, D.sub.c EP, D/CEP, D.sub.c OP, D/COP, D.sub.c EDEP, D/CEDEP, D.sub.c ODEP, D/COEDP, D.sub.c EDEDP, D/CEDEDP, D.sub.c ODEDP and D/CODEDP, wherein (P) represents peroxide treatment. It is pointed out that further process steps such as washing, etc., are understood as being optionally implemented in the total bleaching system. The effluents of each of these stages contain varying amounts and concentrations of the aforedescribed contaminants and the total effluent contamination thereof has presented a particularly difficult disposal problem. Heretofore, waste effluent treatment from CE, CO, D.sub.c E, D.sub.c O, D/CE or D/CO sequence containing bleaching processes, has consisted merely of treating the bulk effluent therefrom with little or no thought given to modification of the bleaching process or separation of the polluting effluents themselves. Accordingly, chemical and physical processes have been heretofore both inadequate in result and economically and commercially inadequate.